Polyfunctional catalysts

ABSTRACT

This invention provides polyfunctional catalysts comprising a composite of platinum and one or more of rhodium, ruthenium and iridium, a substantially larger quantity of one or more base metal oxides in which the metal is selected from the group consisting of metals having an atomic number from 25 to 28 and rhenium, and an alumina support, which composite is made and then deposited on a suitable carrier. In a preferred embodiment, the catalysts contain 1-20 weight percent of said base metal oxide, typically nickel oxide; 0.05-0.5 weight percent platinum; and 0.002-0.3 weight percent rhodium, and an alumina support deposited on a monolith or other carrier. The improved catalysts are especially applicable for purifying exhaust gases from combustion processes, and in particular those from internal combustion engines. 
     These polyfunctional catalysts have in a single formulation, the ability to accomplish four objectives - to oxidize carbon monoxide and unburned hydrocarbons while reducing nitrogen oxides, thereby converting those contaminants found in the exhaust of internal combustion engines into carbon dioxide, water and elemental nitrogen, but without producing significant amounts of hydrogen sulfide, sulfur trioxide or sulfuric acid. Such chemical conversions take place over the catalyst when the ratio of air to fuel supplied to the catalyst is controlled to nearly stoichiometric conditions, thereby maintaining the exhaust feed gases within a narrow compositional range, sometimes called a &#34;window,&#34; in which the catalyst can substantially convert the contaminants.

This application is a continuation-in-part of application Ser. No.608,084, filed Aug. 27, 1975, now abandoned.

BACKGROUND OF THE INVENTION

The invention is broadly concerned with improvements in the field ofcontrolling gaseous contaminants from combustion and in particular fromthe exhaust of internal combustion engines. In particular,polyfunctional catalysts having the ability to convert unburnedhydrocarbons, carbon monoxide, and nitrogen oxides to less harmfulforms, and a method of operating such catalysts are disclosed.

The problem of substantially converting gaseous contaminants orpollutants in automotive exhaust emissions to less harmful forms hasbeen the subject of much research activity, especially in recent years.There are four principal gaseous components of auto exhaust which are ofcurrent interest, namely, unburned hydrocarbons, carbon monoxide,nitrogen oxides, and sulfur oxides. Standards for all these components,except sulfur oxides, have been set by the U.S. Government and which newautomobiles already are required to meet. Until recently, adjustments toengine operating conditions have been sufficient to meet the standards.As these standards become increasingly severe, it has been necessary tointroduce new methods for removing these contaminants. Most recently,catalysts have been used to oxidize the unburned hydrocarbons and carbonmonoxide. In the near future catalysts may be needed to meet morestringent limitations on the nitrogen oxides and sulfur oxides (measuredas sulfates and expressed as sulfuric acid) contained in exhaust gases.Removal of nitrogen oxides is accomplished by reducing the oxides tomolecular nitrogen. Although sulfur dioxide is produced by the enginecombustion process, if the sulfur dioxide is not oxidized to sulfurtrioxide or sulfuric acid, then no sulfates are measured. Inconventional exhaust catalysts which usually operate under oxidizingconditions, nitrogen oxides are not significantly reduced but sulfurdioxide is oxidized and sulfur trioxide is produced. Improved catalystsand/or revised operating conditions are required to remove all three ofthe principal contaminants simultaneously without oxidizing the sulfurdioxide present in exhaust gases.

U.S. Pat. No. 3,331,787 discloses a typical precious metal catalyst(platinum and palladium are preferred) which can be used for oxidationof hydrocarbons and carbon monoxide emitted in auto exhaust. Suchcatalysts are operated with an excess of oxygen present to facilitatethe oxidation process. Even should the free oxygen be limited, at leastsome of the sulfur dioxide present is converted to sulfur trioxide orsulfuric acid. Since removing nitrogen oxides involves the reduction ofnitrogen oxides to molecular nitrogen, such reduction is not favored bythe conditions which are used to oxidize the hydrocarbons and carbonmonoxide to water and carbon dioxide. The two reactions normally requiredifferent conditions. For oxidation, an excess of oxygen should bepresent (fuel-lean), while if nitrogen oxides are to be reduced, it isgenerally necessary to operate with a deficiency of oxygen (fuel-rich).

Various proposals, as represented by U.S. Pat. Nos. 3,565,574 and3,741,725, have been made to use two or more catalyst beds in sequenceto treat exhaust gases (both catalysts being nickel-based in 3,565,574,and platinum-palladium-metal oxide being used in sequence in 3,741,725).Usually nitrogen oxides are reacted first with the exhaust gas beingmaintained fuel-rich, followed by injection of air to create a fuel-leancondition suitable for oxidizing of hydrocarbons and carbon monoxide.

A precious metal catalyst (platinum and rhodium) used solely for controlof nitrogen oxides is disclosed in U.S. Pat. No. 3,806,582. Thiscatalyst is operated in the presence of an added reducing gas. It isintended that this catalyst be applied to nitric acid plant tail gas,where addition of a reducing gas is feasible. This is less practicalduring operation of automobiles and a fuel-rich operation would be usedinstead. However, operating an automobile with an excess of fuel is lesseconomical and produces substantial amounts of unburned hydrocarbons andcarbon monoxide, which must be removed by an oxidation catalyst.

In U.S. Pat. No. 3,840,471 a catalyst comprising platinum and rhodiumalloyed or mixed with a relatively smaller amount of base metal (nickelin the example given) on an inert support ("Torvex" by E. I. duPont deNemours & Co. in the example given) is disclosed which will (i) oxidizehydrocarbons and carbon monoxide, or (ii) reduce nitrogen oxides withthe addition of a reducing fuel. However, the patent does not disclosethe simultaneous removal of all three of these contaminants and suggeststhat if the catalyst is used to remove all components that sequentialoperation such as discussed above would be required. Conditions would beadjusted by adding a reducing gas for removal of nitrogen oxide, oralternatively, by adding air for oxidation of hydrocarbons and carbondioxide.

A previously proposed polyfunctional catalyst, disclosed in U.S. Pat.No. 3,370,914, is capable of removing all three major contaminantssimultaneously. A reduced nickel or alumina catalyst promoted by alkaliand alkaline earth metals is used to cause the exhaust gases to come tochemical equilibrium. It is shown in the patent that if thermodynamicequilibrium could be achieved, the contaminants would be substantiallyremoved. Such a catalyst will promote the equilibrium of the proposedreactions, but it is believed that this catalyst does not retain thisactivity for a commercially practical period of time and the amount ofcatalyst required for automotive use could be excessive for satisfactoryemission control.

In summary, it is known in the prior art (1) to use precious metalcatalysts for oxidizing carbon monoxide and hydrocarbons (U.S.3,331,787), (2) to use precious metal catalysts for reducing nitrogenoxide in the presence, of reducing gas (U.S. 3,806,582), (3) to use analloy or mixture of precious metals and a relatively smaller amount ofbase metals in a catalyst which can be used for either oxidation orreduction when suitable operating conditions are provided (U.S. Pat. No.3,840,471), (4) to adjust exhaust gas compositions so that either theoxidation or the reduction is achieved (U.S. Pat. No. 3,565,574 and3,741,725), and (5) to use a base metal catalyst to promote equilibriumof reactions favorable for removing each of the three principalcontaminants in exhaust gases (U.S. Pat. No. 3,370,974).

Another catalyst is disclosed in U.S. Pat. No. 3,883,444, having thecapability of reacting all three of the major contaminantssimultaneously when stoichiometric amounts of oxygen are present in theexhaust gases. At low space velocities this simultaneous conversion wasfairly complete for a short period of time, but similar results were notobtained at higher space velocities. Palladium alone is used in thecatalysts in combination with large amounts of cobalt and nickel oxides.However, palladium is sensitive to the sulfur and lead contents of thefuel, and has little ability to retain its activity when operated withan engine operated at essentially stoichiometric conditions, and the useof palladium as the only platinum group metal in the catalyst may not besuitable for use where high levels of conversion of contaminants must bemet for extended periods of time.

What has been needed, but not shown in the foregoing prior art, is acatalyst having the ability for a commercially acceptable period tooxidize hydrocarbons and carbon monoxide, without producing significantamounts of sulfur trioxide or sulfuric acid in the exhaust gases or inthe atmosphere into which the gases are discharged, while at the sametime and with the essentially same operating conditions, to reducenitrogen oxides without producing significant amounts of hydrogensulfide, thus avoiding adjusting catalyst operating conditions toproduce separate oxidizing and reducing zones. Such a result has beenacomplished in the present invention by a novel catalyst which is usedin conjunction with an internal combustion engine in which the air-fuelratio is closely controlled. The catalyst of this invention may also beused in plural catalyst operations, for instance, in conjunction with anoxidation catalyst in separate reaction zones.

SUMMARY OF THE INVENTION

The invention comprises polyfunctional catalysts for substantiallyreacting contaminants in the exhaust gases from combustion processesgenerally, and in particular from internal combustion engines, which,when operated under suitably controlled conditions, can simultaneouslyreduce nitrogen oxides and oxidize hydrocarbons and carbon monoxide,without producing appreciable quantities of hydrogen sulfide, sulfurtrioxide or sulfuric acid. These polyfunctional catalysts comprise aplatinum group metal component with the addition of a substantiallygreater quantity of base metal oxides. Such catalysts include as theprecious metal component platinum plus one or more of the platinum groupmetals, rhodium, ruthenium and iridium, including mixtures or alloysthereof. More than two platinum group metals may be present in thecatalysts, for instance, the catalysts may also contain palladium, e.g.platinum, palladium and rhodium. In the preferred embodiment, at leastplatinum and rhodium are used. The base metal oxide may be selected fromthe group consisting of the oxides of metals having an atomic numberfrom 25 to 28, i.e. cobalt, nickel, iron, and manganese, and rhenium.Such base metal oxides are capable of existing in more than oneoxidation state and this ability is believed to underlie the usefulnessof such oxides in the polyfunctional catalysts of the invention.Promoters for such changes in oxidation state may be useful additions tothe catalysts. The catalysts may also contain other base metalcomponents. In a preferred embodiment nickel oxide is used. While theprecious metals are used in small quantities, typically at least about0.02 e.g. 0.05-0.5, weight percent platinum, and at least about 0.001,e.g. 0.002-0.3, weight percent rhodium, ruthenium or iridium or theircombination in a finished catalyst, thus totalling only about 0.021 ormore, e.g. 0.052 to 0.8, percent by weight of precious metals, thequantity of base metal oxide is substantially larger, typicallycomprising 1-20, preferably about 1 to 6, percent by weight of thefinished catalyst. The amounts of platinum group metals and base metaloxide employed may depend, for instance, on the type of carrier on whichthese materials are placed. These active components along with analumina support are deposited on a formed or macrosize carrier, forexample, a monolithic structure such as a ceramic or metal honeycomb ora bed of particulates e.g., small beads or pellets. The channel walls ofmonoliths typically have a film or coating, sometimes called a washcoat, activated coating, or slip, which provides the large B.E.T. areabeneficial as a support for contacting exhaust gases with catalyticallyactive agents. Such coatings, as discussed in U.S. Pat. No. 3,565,830,provide a surface area available for catalyst deposition of about 20m2/gm or more. The uncoated support may often have a surface area ofabout 0.2 to 2 m2/gm. The coatings consist essentially of alumina andare usually applied as a single material or as mixtures whose additionalcomponents are selected from the group consisting of titania, zirconia,silica, magnesia, strontium oxide, calcium oxide, rare earth oxides suchas ceria, lanthanum oxide, and mixtures thereof. The coatings willtypically comprise between 3 and 25 percent by weight of the finishedcatalyst, but where metallic supports are used the coating may compriseas little as 0.2 weight percent of the finished catalyst. In a preferredembodiment a mixture of alumina and ceria is used.

Generally, in accordance with the present invention the catalyst isprepared by combining the platinum group metals and the support byimpregnating the support with the following compounds in aqueoussolution: a compound of platinum and a compound of the base metal; oneor more of a compound of rhodium, a compound of ruthenium, and acompound or iridium. The resulting impregnated support is deposited onthe carrier by contacting the carrier with an aqueous dispersion ofparticles of the impreganted support, which is heated sufficiently toprovide a composite of the platinum group metal component, the basemetal oxide component and the support on the carrier.

The catalysts of the present invention can be made by depositing anaqueous composite or slip of the platinum group metal component, thebase metal component and an alumina support on a monolith or othercarrier. Such procedure is not only practical and economic from thecommercial standpoint, but the resulting catalysts exhibit outstandingperformance characteristics. The platinum group metal component-basemetal component-alumina support slip can be prepared in a variety ofways, including the deposition of the base metal component on thealumina support component followed by deposition of the platinum groupmetal component on the alumina support component. This order of addingthe base metal and platinum group metal components may be reversed, orthe platinum group metal and base metal components may be simultaneouslycontacted with the alumina support component and deposited thereon.Various methods of depositing precious metals are described in U.S. Pat.Nos. 3,331,787 and 3,565,830.

In exemplary procedures for making the catalysts of the invention anaqueous solution of a base metal component, e.g. nickel nitrate, can bemixed with powdered alumina support component and the material dried toremove water. Further heating of the composite, say at calcinationtemperatures, may then be employed to convert the base metal componentto an oxide form deposited on the alumina support. This metal componentis thereby fixed, i.e., placed in water-insoluble form, on the aluminasupport. The resulting material can be mixed with the platinum groupmetal component, e.g. in aqueous form, and the composite may be treatedto fix the platinum group metals on the support. Fixing may be done in avariety of ways such as by treatment with a gas such as hydrogensulfide, or a liquid, such as acetic acid, and/or by reaction of theplatinum group metal components, e.g. a basic platinum compound and anacidic rhodium compound. The latter fixing operation can be facilitatedby adding an acidic component, e.g. glacial acetic acid, to the materialundergoing treatment. The composite of the platinum group metalcomponent, base metal component and alumina support can be comminuted,i.e. reduced in particle size, as by ball-milling. The mixture ofcatalytically promoting components and alumina support is deposited onthe macrosize carrier and the composition is dried, and, if desired,calcined at high temperatures, typically about 500° to 800° C., to yieldof polyfunctional catalyst. The calcination may be conducted in air.

The catalysts of the present invention are preferably manufactured inaccordance with the foregoing procedures by combining a finely-divided,high surface area, alumina support component with one or more aqueoussolutions of the catalytically-promoting metal components, and therebyproviding a composite which has essentially all of the liquid absorbedby the solids. The catalytically-promoting metal components of thesolid, finely-divided composite are converted into an essentiallywater-insoluble form, i.e. fixed, after each or all of these componentsare mixed with the alumina support component. This process can beaccomplished by employing an alumina support component which issufficiently dry to absorb essentially all of thecatalytically-promoting metal components, and thecatalytically-promoting metal components can then be fixed. Duringfixing the composite preferably remains essentially dry, i.e., it hasessentially no unabsorbed liquid present. The composite containing thefixed, catalytically promoting metal components may then be comminutedas a slurry, and the resulting slurry used to coat the carrier. Thecomposite is dried and may be calcined.

The polyfunctional catalysts of the invention when contacting exhaustgases produced by a combustion process and when operated with theair-fuel ratio in contact with the catalysts controlled close to thestoichiometric ratio, are capable of substantially converting all threemajor contaminants without producing significant amounts of hydrogensulfide, sulfur trioxide or sulfuric acid. The air-fuel ratio isadjusted to produce neither a substantially fuel-rich nor asubstantially fuel-lean condition in the exhaust gases taken on anaverage basis. Such a control system will ordinarily result in smallfluctuations within narrow limits about the desired air-fuel ratioalthough there may be short periods of operation outside of the desiredrange. Operating within these narrow limits, termed a "window", theexhaust gases after treatment will have no more than a small amount offree oxygen. With such exhaust gases the polyfunctional catalysts of theinvention will reduce nitrogen oxides and at the same time oxidizecarbon monoxide and hydrocarbons when used to treat automotive engineexhaust gases with the air-fuel ratio in contact with the catalystcontrolled near stoichiometric conditions. Thus the catalysts of theinvention make it possible to meet the stringent exhaust emissionstandards which may be set in the near future.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the following description presents the catalysts and methods ofthe invention in connection with an important application, namely toautomotive use, it will be recognized by those skilled in the art thatthe invention has broader applications to other combustion processeswhere similar problems exist.

Catalysts

A polyfunctional catalyst according to the invention comprises aplurality of platinum group metals which retain a high level ofeffectiveness for commercially useful periods of time by the addition ofa substantially greater quantity of base metal oxide. The platinum groupcatalyst component contains platinum and one or more of rhodium,ruthenium or iridium, preferably at least rhodium. The catalysts maycontain more than two platinum group metals. Preferably platinum andrhodium are used, although other combinations of platinum group metalsmay be used, e.g., platinum, palladium and rhodium. The amount ofplatinum in the catalysts may generally be more than the total of otherplatinum group metals present. Catalysts of the best activity, and whoseperformance characteristics have less dependence on the presence of thebase metal oxide component, contain a weight ratio of platinum to totalrhodium, ruthenium and iridium of about 2 to 5:1, and as this ratioincreases the importance of the base metal oxide component increases.Due to the cost and limited availability of rhodium, ruthenium andiridium, this ratio is often about 8 to 30:1. The ratio of platinum torhodium in a preferred embodiment is approximately 95/5 Pt/Rh. Sincethis ratio is approximately that in which platinum and rhodium are foundin South African ore, there are commercial advantages to using a 95/5ratio. The ore or mine ratio of Pt/Rh/Ir is approximately 19/1/0.2 andthis ratio of metals may advantageously be employed. It may be desirablehowever to increase the amount of rhodium present by a factor of about 2and for instance with a ratio of about 20 Pt/2 Rh/0.3 Ir. However,compositions varying from these ratios are effective and ratios of 95/5to 50/50 Pt/Rh, Ru and Ir may often be employed. In a preferredembodiment, utilizing Pt/Rh on a monolithic support having the aluminawash coat, the platinum content would be within the range of 0.05 to 0.5weight percent of the finished catalyst, and the total rhodium,ruthenium and iridium content would be within the range of 0.002 and 0.3weight percent of the finished catalyst, for a total precious metalcontent between 0.052 and 0.8 weight percent. The amounts of preciousmetals given above are stated in ranges related to their effectivenessand the cost of the finished catalyst used with engines. Catalystshaving precious metal contents outside the preferred range areeffective, even though not preferred for practical reasons. It should beunderstood that the optimum precious metal loading for particularapplications will vary somewhat among the various types of supports,which have different bulk densities and surface areas. Thus, theconcentration of the precious metals may be varied to suit the supportused, while attempting to achieve the same result. However, the totalamount of precious metals used may be related to the piston displacementof the engine and the weight of the automobile rather than the type ofsupport used.

The quantity of base metal oxide used is in general substantiallygreater than that of the precious metal. In general, the weight ratio ofbase metal oxide to precious metal will be at least 2 to 1. In onepreferred embodiment the base metal oxide content is about eight timesthat of the precious metal. Typically the base metal oxide is 1-20percent by weight of the finished catalyst. The base metal oxide may beselected from the group of oxides of metals having an atomic number from25 to 28 and rhenium, i.e. the iron group metals (nickel, cobalt andiron), manganese and rhenium. Such metal oxides can exist in multipleoxidation states and this characteristic is believed to be useful in thecatalysts of the invention. In a preferred embodiment nickel oxide canbe used. The base metal oxide component may be a mixture of oxides suchas those of cobalt and manganese, with or without nickel oxide, cobaltand nickel, manganese and nickel or the like. Promoters for changes inthe oxidation state of metals may be useful additions to the catalyst.

Alumina coatings which provide a large surface area for the catalyticcomponents therein are typically applied to those carriers which have arelatively low surface area, particularly monoliths. Reference can bemade for more details to U.S. Pat. No. 3,565,830. Where used withmonoliths, the surface area available for catalyst deposition can beincreased from a total surface area of say 0.2 to 2 m² /gm for theuncoated support to about 20 m² /gm or more. Such coatings are composedessentially of alumina and may be applied in combination with compoundsselected from the group consisting of titania, zirconia, silica,magnesia, calcium oxide, strontium oxide, rare earth oxides e.g., ceriaand lanthanum oxide, and mixtures thereof. The coatings will typicallybe present in the range of 3 to 25 percent by weight of the finishedcatalyst where a refractory oxide monolith is used, but may comprise aslow as 0.2 weight percent where the support is a metallic structure.

Embodiments of suitable preparations for formulating polyfunctionalcatalysts of the invention are illustrated in the following Examples.

EXAMPLE I Pre-impregnation of Wash Coat--With Sulfiding

Five hundred (500) grams of a powdered mixture of 90% alumina and 10%ceria by weight is impregnated by mixing it in a mechanical mixer with195 grams of nickel nitrate hexahydrate which has been dissolved in justenough water to completely saturate the powder. Then, the wet powder isdried and calcined for two hours at 650° C. Three hundred grams of thecalcined powder is then impregnated with 160 milliliters of an aqueoussolution containing 10.3 grams of chloroplatinic acid and 0.595 grams ofrhodium chloride. The wet powder is placed in a chamber, evacuated, andtreated with hydrogen sulfide at room temperature to fix the preciousmetals in place. Thereafter, the sulfided powder is washed withdeionized water to free it of chlorides, dried at 125° C., and calcinedat 500° C. for two hours. The resulting powder is then ballmilled for 19hours along with 400 milliliters of deionized water and six millilitersof 15 N nitric acid to reduce the particle size. A monolithic support ofcordierite-mullite made by the Technical Ceramics Products Division ofthe 3M Company (AlSi Mag 795) is then dipped into the milled slurry tocoat it with the pre-impregnated slurry to a concentration of 2grams/in³. Excess slurry is blown off by compressed air and the supportis dried at 125° C. to remove free water and calcined at 500° C. toyield a finished polyfunctional catalyst having the composition 0.275weight percent Pt, 0.0145 weight percent Rh, and 2.0 weight percent Ni₂O₃. Another catalyst having the composition 0.2 weight percent platinum,0.005 weight percent palladium, 0.005 weight percent rhodium and 2.25weight percent Ni₂ O₃ can be similarly made by including in thepreparation corresponding amounts of platinum, rhodium and palladiumchlorides and nickel nitrate hexahydrate.

EXAMPLE II Pre-impregnation of Wash Coat--Without Sulfiding

Three thousand (3000) grams of a powdered mixture of 90% alumina and 10%ceria by weight is impregnated with an ammoniacal solution containing884 grams of nickel formate in just enough water to completely saturatethe powder. Then, the wet powder is dried and calcined for 2 hours at650° C. The calcined powder is then impregnated with an aqueous-aminesolution containing 43.4 grams of platinum as H₂ Pt(OH)₆, followed by anaqueous solution containing 14.4 grams of rhodium as rhodium nitrate,the precious metal solutions containing insufficient water to completelysaturate the powder, and finally 360 milliliters of glacial acetic acidwhich fixes the platinum. The resulting slurry is stirred for 30 minutesand sufficient deionized water added to reduce the solids content to46%. The slurry is then ballmilled for 19 hours to reduce the particlesize. A monolithic support of cordierite-mullite of the same type usedin Example I is then dipped into the milled slurry to coat it with thepre-impregnated slurry to a concentration of 2 grams/in³. Excess slurryis blown off by compressed air and the support is dried at 125° C. toremove free water and calcined at 500° C. to yield a finishedpolyfunctional catalyst having the composition 0.217 weight percent Pt,0.072 weight percent Rh, and 2.0 weight percent Ni₂ O₃.

EXAMPLE III Pre-impregnation of Wash Coat--Mixed Base Metal

Two thousand one hundred (2,100) grams of a powdered mixture of 90%alumina, 10% ceria by weight is impregnated with an aqueous solutioncontaining 932 grams of cobalt nitrate hexahydrate and 1,200 grams of a50% aqueous solution of manganese nitrate which contains just enoughwater to completely saturate the powder. The mixture is dried andcalcined for 2 hours at 650° C. The calcined powder is then impregnatedwith an aqueous amine solution, containing 25.48 grams of platinum as ahydrate platinum (IV) oxide and 2.32 grams of rhodium as rhodiumnitrate. The precious metal solutions contain insufficient water tocompletely saturate the powder. Then 240 milliliters of glacial aceticacid is added which fixes the platinum. Water is added to form a slurrycontaining 49 percent solids and the slurry is ballmilled for 19 hoursto reduce the particle size. A monolithic cordierite support is thendipped into the slurry to coat it with the pre-impregnated slurry to aconcentration of 2.17 g/in³ after drying and calcining at 500° C. Thefinished polyfunctional catalyst has a composition of 0.167 weightpercent platinum, 0.015 weight percent rhodium, 1.57 weight percentcobalt oxide and 1.57 weight percent manganese oxide.

Operation of the Catalysts

A polyfunctional catalyst according to the invention is capable ofreducing nitrogen oxides and at the same time oxidizing hydrocarbon andcarbon monoxide. The ratio of the amounts of air and fuel supplied tothe catalyst affects the amounts of contaminants produced. In order tomeasure catalyst performance with precision an exhaust gas mixturecorresponding to a given air-fuel ratio was formulated to include allthe major components expected in such exhaust gas mixtures. Thesemixtures were contacted with the catalysts and the percentage conversionmeasured. An air-fuel ratio of 14.65 (wt. basis) is the stoichiometricratio corresponding to the combustion of a hydrocarbon fuel with anaverage formula CH₁.88. Fuels with different carbon/hydrogen ratios willrequire slightly different air-fuel ratios to produce a stoichiometricmixture. To avoid confusion in making comparisons, the Greek symbol λhas been used at times to represent the relationship of a particularair-fuel ratio to the stoichiometric ratio. The air-fuel ratio isdivided by the stoichiometric ratio so that in this system λ=1 is astoichiometric mixture, λ>1 is a fuel-lean mixture and λ<1 is afuel-rich mixture. For example, at an air-fuel ratio of 14.5, theλ=14.5/14.65=0.9898.

Conversion is substantially complete with fresh catalysts of theinvention, usually 90-100% of each contaminant may be removed by thepolyfunctional catalysts when operated within narrow limits near thestoichiometric air-fuel ratio. If the fuel-rich condition (below 14.65or λ<1) is used, nitrogen oxides are reduced more favorably, and if afuel-lean conditon (above 14.65 or λ>1) is used, carbon monoxide andhydrocarbons and oxidized more favorably. Although the catalysts couldbe operated to remove only one type of contaminant from exhaust gases,it is a distinctive feature of the catalysts of this invention that theycan convert all three contaminants to harmless compounds when operatingin a narrow range of air-fuel ratios or "windows," close to thestoichiometric air-fuel ratio, without producing significant amounts ofhydrogen sulfide, sulfur trioxide or sulfuric acid and from the sulfurin the fuels used in operating engines which are the source of theexhaust gases. Bounds for such windows are established in general byair-fuel ratios where conversion of one or more of the contaminantsreaches only a minor value. All of the contaminants can be substantiallyremoved if the air-fuel ratio is closely controlled within a "window"having boundaries of about 14.4 and 14.6, although it may be feasible tooperate in the range of 14.2 to 14.9 depending to a large extent uponsuch factors as fuel composition. It is feasible to control thevariations in air-fuel ratio within hese average limits. For example,the fuel supply system can be controlled by an oxygen sensor located inthe exhaust gases. The normal variations of such a control systemprovide a continuous swinging of air-fuel ratio about the desired value,near the stoichiometric value. The variations are small, however, andthe average air-fuel ratio generally remains within the operatingwindow, and short excursions outside the window may not unduly adverselyaffect the operations. Under such conditions, it has been found that thecatalysts can remove substantially all of the three contaminants. Shoulda significant excursion outside the operating window occur, the catalystwill be capable of converting more favorably whichever of thecontaminants the conditions favor, that is, either nitrogen oxides whenthe air-fuel ratio is fuel-rich (λ<1) or the carbon monoxide andhydrocarbons when the air-fuel ratio is fuel-lean (λ<1).

Another benefit of operating a polyfunctional catalyst of the inventionwith the air-fuel ratio controlled close to the stoichiometric value isthat essentially no hydrogen sulfide sulfur trioxide or sulfuric acidmay be produced by oxidation of sulfur dioxide present in the exhaustgases. A vehicle so equipped can be expected to meet governmentstandards for sulfate emissions which may be established since suchemissions are related to the amount of sulfur dioxide oxidation whichoccurs.

It should be noted that although one hundred percent conversion is adesirable goal, it is not required. The governmental standards for themaximum allowed exhaust gas emissions are expressed as grams per mile avehicle travels. If the raw exhaust gases leaving an engine's exhaustmanifold contain a high concentration of contaminants, a largepercentage conversion of those contaminants will be required. On theother hand, if the contaminants in the raw exhaust gases are already atlow concentrations, then only a moderate percentage conversion will beneeded to meet the standards.

The polyfunctional catalysts of the invention promote extremely highconversions of all three principal contaminants within the rangeaveraging 14.4 to 14.6 or slightly higher air/fuel ratio. The air/fuelrange, "window," for the catalysts of the invention is rather narrow,requiring close control of the air/fuel ratio. The target operatingpoint would appear to be on the fuel-rich side of stoichiometricconditions, that is, λ<1. No single steady state conditions can bechosen which will provide the best possible conversion of all thecontaminants. It will be appreciated, however, that normal variation inair/fuel ratio in an oxygen sensor controlled engine will beapproximately ±0.1, or ±0.3, air/fuel units (average weight basis) orless, except for short excursions outside these ranges. Under suchconditions the air/fuel ratio is constantly charging and, owing tomixing of gases and the residence time in the exhaust system, theaverage air/fuel ratio may be at about the midpoint of the window.

It should be noted that since the catalysts have been found to be activefor both oxidation and reduction, it is within the scope of theinvention to use the catalysts in two stages operating sequentially,such as has been disclosed in the prior art. For example, reducingconditions would be established (an air/fuel ratio λ<1 by, for instance,adding a fuel component to have slightly reducing conditions) forreducing nitrogen oxides, then followed by creation of an oxidizingcondition by injection of air for oxidizing hydrocarbons and carboonmonoxide. Alternatively, the reducing and oxidizing stages could bereversed. While such a sequential operation is not the preferredembodiment, it is a feasible method of utilizing the present catalystsif, for example, precise air/fuel ratio control is not available. Alimitation on the application of some of the polyfunctional catalysts ofthe invention in this manner is the possible formation of ammonia in thefirst stage which is oxidized to nitrogen oxides in the second stage. Insuch applications, catalyst compositions should be chosen to minimizeammonia formation under reducing conditions.

Closely controlling air/fuel ratios near λ=1 has been found to be aprecise operating condition. Prior art catalysts which have goodresistance to poisoning when operated as oxidizing catalysts (λ>1)rapidly lose activity when essentially no oxygen is present in theexhaust gases. The catalysts of the present invention, however, have asubstantial ability to retain adequate performance during use. Theconversion of carbon monoxide and nitrogen oxides may not besignificantly changed during substantial use. Although the catalysts maylose some of their effectiveness for hydrocarbon removal, they remainsatisfactory. The significance of such results is that conversion ofeach of the principal contaminants of about 70% is needed to meetgovernmental standards for automotive emissions. The catalysts of thepresent invention, operating under identical conditions, are asubstantial improvement over catalysts previously available.

It should be pointed out that the conditions under which the catalystsof the invention are tested are typical of those found in the averageautomobile exhaust systems. However, the conditions vary widely,depending on load on the engine. The amount of contaminants also changessubstantially as engine conditions vary. Fresh catalysts aresubstantially more active than "aged catalysts" operating at about 650°C. and 100,000 VHSV (volume hourly space velocity), however, it is theperformance of such aged catalysts which indicates their real value forcommercial applications. The catalysts may operate at about 400° to 800°C., usually about 450° to 700° C.

A prior art catalyst in which only platinum-rhodium were used as theprincipal catalytic agents, and thus having no base metal oxide, hadgood initial activity, but substantially inferior performance when aged.Removal of nitrogen oxides and hydrocarbons never approaches 100%, evenunder the most favorable conditions outside the operating window.Performance of the catalyst for CO conversion is better but not as goodas the catalysts of the present invention.

Another prior art catalyst was tested. This catalyst usesplatinum-rhodium and a base metal, the three elements being alloyed toform the catalyst. This catalyst is similar to that disclosed in U.S.Pat. No. 3,840,471 in that only a minor percentage of base metal isused. The catalyst was prepared in a similar manner to the Example inthe patent, deleting the separate nickel impregnation, but includingnickel nitrate in the precious metal solution. The catalyst containedabout 0.1 weight percent Pt, 0.017 weight percent Rh, and 0.029 weightpercent Ni. It is believed to be of significance that the base metal isbeing alloyed by depositing with the precious metal and being reduced tothe metallic state rather than being separately deposited and oxidizedas in the present invention. The performance of the catalyst of thisprior patent is somewhat similar to that of the prior art catalystmentioned above in which only platinum and rhodium were used ascatalysts. However, the position of the window is different for the twocatalysts. Removal of nitrogen oxides is better with the alloy catalystthan with the catalyst containing only precious metals as the principalcatalytic agents, but the inferior removal of hydrocarbons and carbonmonoxide is present again.

Performance of these prior art catalysts after aging, shows serious lossof activity for conversion of nitrogen oxides and hydrocarbons, evenunder the most favorable conditions. Such results effectively define thecatalysts' performance since the conversion of CO is significantlyhigher. The loss of conversion of nitrogen oxides and hydrocarbons alsoresults in much lower crossover point for these conversions, indicatingthat these catalysts are inferior for use as polyfunctional catalystscompared to catalysts of the present invention which retain higheractivity after the same aging.

When the catalysts of the present invention are used in an automobile,the performance is reported according to U.S. Government standards ingrams of each component discharged for each mile traveled over aprescribed sequence of operation. In tests using a four cylinderautomobile, after 4,000 miles of air/fuel ratio-controlled operation thecatalysts are capable of giving results similar to the following:

    ______________________________________                                        Hydrocarbons -     0.22 grams/mile                                                               0.21 grams/mile                                            Carbon monoxide -  1.93 grams/mile                                                               1.41 grams/mile                                            Nitrogen oxides -  0.87 grams/mile                                                               0.94 grams/mile                                            Sulfates -         0.0033 grams/mile                                                             0.00054 grams/mile                                         ______________________________________                                    

It should be noted that the quantities of sulfates measured areconsidered negligible. The quantities of hydrocarbons, carbon monoxide,and nitrogen oxides may be compared with the most stringent Federalstandards anticipated for 1978 of 0.41 gram/mile hydrocarbons, 3.4grams/mile carbon monoxide, and 0.4 gram/mile nitrogen oxides. Morerecently, it appears that these standards will take effect in 1980,except that the nitrogen oxide emissions will be 2 grams per mile until1981 at which time the maximum allowable will be 1 gram per mile. Thenitrogen oxide emissions in the above tests could be lowered further bydecreasing the air-fuel ratio, possibly at the expense of increasedhydrocarbon and carbon monoxide concentrations. Alternatively, allemissions could be reduced by increasing the amount of catalyst used.

It is claimed:
 1. A catalyst suitable for simultaneously oxidizinggaseous hydrocarbons and carbon monoxide and reducing nitrogen oxides,consisting essentially of(a) a support consisting essentially ofalumina; (b) platinum group metal component, the platinum group metalbeing selected from the group consisting of platinum plus at least oneof rhodium, ruthenium, iridium and mixtures thereof, and alloys of saidplatinum group metals; (c) a base metal oxide component, the base metalbeing selected from the group consisting of metals having an atomicnumber from 25 to 28 and rhenium, and mixtures thereof, said base metaloxide being present in a weight ratio of said platinum group metalcomponent of at least 2 to 1 and; (d) a carrier on which said support,composited with both said platinum group metal component and said basemetal oxide component are deposited; (e) said catalyst being prepared bycombining said platinum group metal component, said base metal oxidecomponent, and said support by impregnating said support in particulateform with the following compounds in aqueous solution: a compound ofsaid base metal, a compound of platinum; and one or more of a compoundof rhodium, a compound of ruthenium, or a compound of iridium; theresulting impregnated support being deposited on said carrier bycontacting said carrier with an aqueous dispersion of particles of saidimpregnated support, and heating said impregnated support at atemperature sufficiently high to provide on said carrier a composite ofsaid platinum group metal component, said base metal oxide component andsaid support.
 2. The catalyst of claim 1 wherein said platinum groupmetal consists essentially of platinum and rhodium, and optionally,palladium.
 3. The catalyst of claim 1 wherein the carrier is monolithic.4. The catalyst of claim 1 wherein the base metal oxide consistsessentially of nickel oxide.
 5. The catalyst of claim 4 wherein theplatinum group metal consists essentially of platinum and rhodium, andoptionally palladium.
 6. The catalyst of claim 5 wherein platinum is0.05 to 0.5 weight percent and rhodium is 0.002 to 0.3 weight percent ofthe catalyst, the amount of platinum is at least equal to the amount ofrhodium, and the nickel oxide is 1 to 20% of the catalyst.
 7. Thecatalyst of claim 6 wherein the carrier is monolithic.
 8. The catalystof claim 7 wherein the support consists essentially of aluminastabilized by ceria.
 9. A catalyst suitable for simultaneously oxidizinggaseous hydroncarbons and carbon monoxide and reducing nitrogen oxides,consisting essentially of(a) a support consisting essentially ofalumina: (b) a platinum group metal component, the platinum group metalbeing selected from the group consisting of platinum plus at least oneof rhodium, ruthenium, iridium, and mixtures and alloys thereof, saidplatinum group metal component being deposited on said support by mixingsaid support in particulate form with compounds of said platinum groupmetals in solution in an aqueous medium, said compounds of said platinumgroup metals essentially including a compound of platinum and at leastone of a compound of rhodium, a compound of ruthenium and a compound ofiridium, said support and aqueous medium being present in amounts suchthat essentially all of said aqueous medium is absorbed by said support,and fixing said platinum group metal on said support; (c) a base metaloxide component, the base metal being selected from the group consistingof metals having an atomic number from 25 to 28 and rhenium, andmixtures thereof, said base metal oxide component being present in aweight ratio to said platinum group metals of at least 2 to 1 and beingdeposited on said support by mixing said support in particulate formwith an aqueous solution containing a compound of said base metal, saidsupport and aqueous solution being present in amounts such thatessentially all of said aqueous solution is absorbed by said support,and fixing said base metal as an oxide on said support; (d) a carrier onwhich said support, composited with both said platinum group metalcomponent, and said base metal oxide component are deposited as definedhereinbelow; (e) said catalyst being made by contacting said carrierwith an aqueous dispersion of particles of said impregnated support andheating said impregnated support at a temperature sufficiently high toprovide on said carrier a composite of said platinum group metalcomponent, said base metal oxide component and said support.
 10. Acatalyst of claim 9 in which said fixings of said platinum group metalsand said base metal are conducted in the absence of unabsorbed aqueousmedium.
 11. A catalyst for simultaneously oxidizing gaseous hydrocarbonsand carbon monoxide and reducing nitrogen oxides consisting essentiallyof(a) a support consisting essentially of alumina; (b) a platinum groupmetal component, the platinum group metal being selected from the groupconsisting of platinum plus rhodium, mixtures and alloys thereof, saidplatinum group metal component being about 0.05 to 0.8 weight percent ofthe catalyst and the amount of platinum being at least equal to theamount of rhodium, said platinum group metal component being depositedon said support by mixing said support in particulate form with acompound of platinum and a compound of rhodium in solution in an aqueousmedium, said support and aqueous medium being present in amounts suchthat essentially all of said aqueous medium is absorbed by said supportand fixing said platinum group metal on said support in the absence ofunabsorbed aqueous medium; (c) nickel oxide being 1 to 20 weight percentof the catalyst and deposited on said support by mixing said support inparticulate form with an aqueous solution containing a compound ofnickel, said support and aqueous solution being present in amounts suchthat essentially all of said aqueous solution is absorbed by saidsupport, and fixing the nickel of said nickel compound as nickel oxideon said support in the absence of unabsorbed aqueous medium; and (d) amonolithic carrier on which a composite of said support, composited withboth said platinum group metal component and said nickel oxide aredeposited as defined hereinbelow; (e) said catalyst being prepared bycomminuting said support containing said platinum group metal componentand said nickel oxide in an aqueous medium to provide an aqueous slurry,and contacting the resulting slurry with said carrier and heating saidslurry at a temperature sufficient by high to provide on said carriersaid composite.
 12. The catalyst of claim 11 wherein said supportconsists essentially of alumina stabilized by ceria.
 13. The catalyst ofclaim 1 wherein said support consists essentially of alumina and ceria.14. The catalyst of claim 1 wherein said impregnated support is heatedon said carrier to drive off free water and then is calcined on saidcarrier.
 15. The catalyst of claim 1 wherein the step of impregnatingsaid support with said compounds in aqueous solution is followed by astep of drying and calcining the resulting impregnated support prior tothe steps of forming said aqueous dispersion of particles of saidimpregnated support and contacting said carrier with said aqueousdispersion.
 16. The catalyst of claim 15 wherein the base metal oxideconsists essentially of nickel oxide, the platinum group metal consistsessentially of platinum and rhodium, and optionally palladium, platinumis 0.05 to 0.5 weight percent and rhodium is 0.002 to 0.3 weight percentof the catalyst, the amount of platinum is at least equal to the amountof rhodium, and the nickel oxide is 1 to 20 weight percent of thecatalyst.